1. Field of the Disclosure
The present application relates to flexible circuit boards, and in some embodiments, to ultra-thin flexible circuit boards and their fabrication methods.
2. Background
A polyimide coverlay is conventionally used in a print circuit board (PCB) to cover and protect metal circuits formed thereon. As technology advances, the printed circuit board becomes increasingly thinner, lighter and multi-functional. The thinner dimension of the printed circuit board usually requires an ultra-thin polyimide coverlay.
In the fabrication of a flexible printed circuit board, a printed circuit may also be formed on a polyimide film used as a carrier. However, technical difficulties may arise in the fabrication steps when the polyimide film becomes very thin (less than 6 μm).
Ultra-thin polyimide films are difficult to fabricate with current processing methods. Some polyimide films currently available on the market may have a thickness less than 10 μm. However, polyimide films with a thickness less than 5 μm are usually not subjected to biaxial orientation, because the stretching process may break the polyimide film. Moreover, the fabrication of the current ultra-thin polyimide films generally does not consider difficulties that may arise in the application of the polyimide film as a carrier of a flexible circuit board, which may require applying an adhesive on the ultra-thin polyimide film. When the thickness of the polyimide film is less than 6 μm, certain processing steps (such as adhesive coating) applied on the polyimide film are very difficult to perform.
Accordingly, there is a need for polyimide films that are convenient to process, and are suitable for use in fabricating ultra-thin flexible circuit boards.